This invention relates to an improved machine for folding sheets of various sizes and materials, such as the paper sheets that make up a book.
As is well known, the several processing steps which result in the formation of a book, booklet, or the like, include folding of the previously printed sheets in two overlapping flaps over each page, such as to define two or more page sheets. This folding operation is advantageously carried out on an automatic machine, of a type well known in the art, which comprises means adapted for folding the sheets in flaps and effective, in actual practice, to push the same along shaped paths such as to cause the sheets to fold over themselves, as well as rollers which act on the sheets already folded into flaps to squeeze them such as to create a neat and permanent crease or fold line.
These rollers are a source of serious technical problems in the construction of said machines for sheet folding, because the requisites imposed on them are both numerous and fundamental. In fact, they are required to be of generous dimensions, in the longitudinal direction, such as to allow the passage therebetween of sheets which may be very large; they must be capable of withstanding considerable pressure stress without flexing; they have to be manufactured to very close tolerance limits along their diametrical dimension, and to a virtually perfect roundness, as otherwise both the pressure applied and the tangential velocity would vary locally, and the sheets would be entrained in an uneven manner; and finally, they are to be so installed in the machine as to hold the pressure exerted by them at a strictly predetermined level in accordance with the type of sheets being processed, and such as to be easily disassembled and reassembled for servicing purposes.
Currently, these requirements are met by fabricating such rollers of solid steel, surface ground.
Furthermore, said rollers are manufactured with integral end hubs intended for pivotal insertion in the stationary frame of the machine. Provision is also made for at least one roller in a pair of opposed rollers to be detachable, or removable, from the other; in other terms, the hubs of one roller are associated with grooves or guiding members which allow for a clearly defined movement of the rollers away from each other against the bias of specially provided pusher members, usually calibrated springs.
This technique, in addition to being expensive, owing to the need for precision grinding rollers of considerable length, also originates serious problems of dynamic balance of the rollers during the machining thereof. In fact, the rollers are rotated about their own axes at a high angular velocity, which induces in the same, due to the presence of unavoidable uneven areas in the distribution of weights, oscillatory and whip phenomena of a more or less enhanced character, owing to which the rollers undergo alteration of their strictly linear pattern between their ends, and flex. It is evident that such dynamic stability deviations can be very serious, in that they cause uneven entrainment of the sheets.
The cited phenomena are further enhanced, or at least undiminished, by the simple roller supports, said rollers being inserted with their hubs free and slidable within the frame. In fact, with the hubs being simply supported for rotation, maximum freedom is conceded to the development of such dynamic oscillatory phenomena and the rollers are allowed to flex in all directions at their middle portion, without said hubs opposing any resistance thereto.
Such drawbacks are further aggravated by the rollers being not only rotated at a very high speed about their axes, but also flexure stressed by the sheets being inserted therebetween under high pressure force.
The above not only applies to the stationary rollers, but also to the raisable ones. In fact, the sheets, as they enter the nip between two rollers, tend to raise the movable roller in a random, within limits, direction, usually not exactly coincident with the direction of upward movement provided for the roller. That is, the movable roller is usually raised somewhat obliquely to the direction of the impact force generated by the admission of a folded sheet, thereby it is also subjected to efforts and flexure with the end supports.
Lastly, it should be noted that the calibrated springs, as employed of usual, in mutually biasing the rollers against each other, slightly yield in time, often to an uneven extent, thus permitting irregular oscillation of the roller ends, instead of, as required, just equal movements of the rollers to and from each other.
The Applicant has found that such drawbacks cannot be even alleviated by coating the rollers with an elastic material capable of accommodating thickness irregularities in the sheets, or the sheet insertion and extraction operations. It could be ascertained, in fact, that this remedy is unacceptable owing both to the material wearing out very rapidly and to its enhancing the irregularities and dynamic vibrations of the rollers due to its elastic action.
The one improvement achieved in relation to the above has been the coating of the rollers either completely, or partially, or along helical paths, with hard rubber, which wears slowly and has no elastic effects. However, this only provided improvement as relates to the frictional action applied by the rollers to the paper sheets, since the harder the rubber is, the less capable is this of absorbing the sheet impact forces, and the more elastic is the rubber, within the limitations imposed by the wear requirements, the greater are the periodic oscillations and prolonged vibrations between the rollers.
Thus, the problems connected with the structure and support of said rollers in the machine are still without a satisfactory solution.